In a typical paper making process, water is extracted from the paper web in a number of different operations. For example, in the forming section of the Fourdrinier, the fiber and water slurry is conveyed on a bronze or synthetic wire across a series of spaced transversely extending foil blades. Each foil blade is provided with a generally sharp edge facing in an upstream direction. As the paper web passes across the foil blade, a vacuum is created between the paper web and the upper surface of the foil blade, and draws water from the paper web. The edge of the next succeeding blade riding against the lower surface of the bronze or synthetic wire functions to remove more moisture.
The paper press felt is used in the press section of a paper machine to support the paper web as the web is passed through a pair of cooperating press rollers to extract water from the web. Following the pressing of the paper web, it is separated from the felt and passed to further processing stations.
While the felt is extracting water from the paper web in the press section, water migrates from the paper web into the felt. The paper stock slurry being treated typically consists of a mixture of wood pulp, pitch, water and numerous chemicals. Some components contained in the paper stock slurry can be detrimental to the efficient operation of the paper press. For example, some components can become attached to and imbedded in the felt and can inhibit the free movement of water from the paper web to the felt.
The felt normally comprises an endless fabric so that after it is separated from the paper web following pressing, the felt is then caused to transverse several return rolls. Thereafter, it is directed back to the point where the wet paper web coming from the wire is received onto the felt. In order to assure uniform porosity of the felt and proper and efficient operation of the press section, the moisture must be uniformly extracted from the felt before the wet paper web is received back onto the felt. It is desirable for the felt be cleaned on its return run after the paper web is separated from the felt. Such cleansing is commonly accomplished by conventional Uhle boxes over which the felt is caused to travel on its return run. During the endless travel of the felt in the press section of the paper machine, the web-supporting surface of the felt is commonly passed in direct contact with one or more Uhle boxes.
The typical suction or Uhle box includes a rigid cover which supports the felt that carries the paper web, and the cover is provided with a plurality of perforations or slots. The cover portion is attached to a vacuum source which functions to remove moisture from the felt by creating a vacuum through the vacuum slots provided in the interior portion of the Uhle box.
Of particular importance in the case of most paper presses is that a build up of pitch typically occurs on the Uhle box strips. Pitch deposition can be detrimental to the efficient operation of the press section. Pitch can deposit on process equipment and press felt resulting in operational problems in such systems. Deposition of pitch can occur not only on metal surfaces in the system, but also on plastic and synthetic surfaces such as machining wires, felts, foil blades, as well as Uhle box strips. Pitch deposits may also break off the surfaces of Uhle box strips in the system and cause spots and defects in the final paper product which ultimately decreases the paper's quality. Machine breaks may also occur. Pitch deposition can also cause premature wear of the wet felts, thereby requiring the costly replacement of such felts.
Typically, the prior art Uhle box includes high density polyethylene or ceramic strips over which the paper press felt travels before entering the vacuum zone that extracts the moisture from the felt. A disadvantage of the use of polyethylene or ceramic strips in the conventional Uhle box is that a shearing force results between the traveling felt and the Uhle box strips. The shearing force arises due to the stress resulting from the applied force which causes the felt to slide relative to the Uhle box strips in a direction substantially parallel to their plane of contact. This resulting shearing force can cause a build-up of pitch which tends to deposit upon the Uhle box strips and the felt. These pitch deposits ultimately reduce the felt life, plug the felt, and accelerate wear of the strips of the Uhle box. To obviate this problem, it is necessary to periodically clean the Uhle box or the individual foil blades to remove pitch deposits which would otherwise build up and commonly obstruct the vacuum slots in the Uhle box or cause damage to the felt. The frequency of the periodic cleaning obviously causes downtime of the machinery and thus substantial production problems, and increased manufacturing costs.
Accordingly, it is an object of the subject invention to overcome the shortcomings of the prior art devices and to provide a new and improved Uhle box for a press section of a paper machine.
Another object of the subject invention is to provide a new and improved Uhle box which reduces pitch deposition in the press felt and on Uhle box strips.
Still a further object of the subject invention is to provide a new and improved Uhle box which provides for more efficient and better felt cleaning, and more efficient moisture removal.
It is another object of the subject invention to provide a new and improved Uhle box which enables a press section to operate more efficiently at lower cost.
A further object of the subject invention is to provide a new and improved Uhle box which results in less wear to the wet felt.
Still another object of the subject invention is to provide a new and improved Uhle box which eliminates the use of foil blades as part of the Uhle box.